Output driver circuit

ABSTRACT

The present invention relates to an output driver circuit for a semiconductor memory device, in particular, a memory device using a DDR II concept or a concept similar thereto, which can reduce a variation in the slew rate of an output driver thereof between maximum and minimum values, while satisfying requirements of characteristics associated with slew rate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an output driver circuit, and moreparticularly to an output driver circuit for outputting internal datafrom a chip in a dynamic random access memory (DRAM) memory device tothe outside of the chip.

2. Description of the Related Art

Semiconductor devices use an output driver circuit to output internaldata from a chip thereof to the outside of the chip through an outputterminal, that is, an output pad.

Such an output driver circuit includes a push-pull driver. Importantfunctions of such a push-pull driver are to adjust the driving strengththereof to a desired level, and to control the slew rate of an outputsignal.

“Driving strength” is a force for driving an output signal toward aparticular voltage level (for example, level of supply voltage or groundvoltage). When the driving strength is higher, the output signal has avoltage level more approximate to the particular voltage level. Such adriving strength may be a pull-up driving strength or a pull-downdriving strength. The pull-up driving strength is a force for driving anoutput signal toward a high voltage level, for example, a supply voltagelevel, whereas the pull-down driving strength is a force for driving anoutput signal toward a low voltage level, for example, a ground voltagelevel. In order to control the voltage level of a signal outputted froman output driver to a desired level, it is necessary to adjust thedriving strength of the output driver.

“Slew rate” means a variation rate of the voltage level of an outputsignal, so that it may be considered as a gradient of voltage to time.Such a slew rate may be a rising slew rate or a falling slew rate. Therising slew rate represents the gradient of an output voltage, the levelof which is transited from a low level to a high level. On the otherhand, the falling slew rate represents the gradient of an outputvoltage, the level of which is transited from a high level to a lowlevel. At a higher slew rate, the gradient of the output voltage issharper. In other words, the level of the output voltage is abruptlyvaried within a short time.

Where an output push-pull driver exhibits a high slew rate, large noisecurrent is generated, even though there is an advantage in terms of dataskew. On the other hand, a low slew rate causes an increase in theamount of data skew, even through there is a reduction in noise current.Accordingly, it is necessary to appropriately adjust the slew rate ofsuch an output push-pull driver to a desired value. For instance, inassociation with double data rate (DDR) II synchronous DRAMs (SDRAMs),standard values of slew rates of output drivers have been established,and strictly managed.

DDR SDRAM was introduced as a substitute for SDRAM in accordance with anew standard scheme to overcome limitations of SDRAM caused by thecontinuously increased bus speed. Such a DDR SDRAM is approximatelysimilar to a standard SDRAM in terms of operation. However, the DDRSDRAM performs transmission two times per cycle (at rising and fallingedges of a clock signal) to achieve a double increase in bandwidth.

Meanwhile, such a DDR SDRAM technique advanced to development of a DDRII SDRAM. For an application thereof to such a DDR II SDRAM, outputdriver circuits additionally have an off-chip driver (OCD) function toperform an increase or decrease in output driving strength in astep-wise fashion so that adjustment of impedance is achieved inaccordance with such output driving strength control. The purpose ofsuch an OCD function for performing control of output driving strengthin an output driver circuit is to achieve impedance matching of theoutput driver circuit to a data output (Dout) signal representinginternal data of a memory device, to which the output driver circuit isapplied. Hereinafter, this operation will be described in brief.

A pre-driver outputs a plurality of data signals, based on the internaldata, that is, the Dout signal. Two of the data signals are applied torespective gates of pull-up-driving and pull-down-driving switchelements, which may be PMOS and NMOS transistors. Another two of thedata signals are applied to an OCD pre-driver. In addition to the lattertwo data signals, the OCD pre-driver receives control signals generatedfrom a control unit of the memory device in accordance with codinginformation of the internal data. Based on the received signals, the OCDpre-driver generates a plurality of up-driving control signals forcontrol of up-driving impedance, and a plurality of down-driving controlsignals for control of down-driving impedance. The up-driving anddown-driving control signals from the OCD pre-driver are inputted to apush-pull driver, which also receives the data signal from thepre-driver. Based on the received signals, the push-pull drivergenerates an output signal having a predetermined strength. In thiscase, the up-driving and down-driving control signals control ON/OFF ofthe pull-up-driving and pull-down-driving switch elements in thepush-pull driver, thereby controlling the impedance of an output driver,that is, the push-pull driver.

However, such a conventional output driver circuit has a problem in thatit is impossible to satisfy requirements of characteristics associatedwith slew rate due to the addition of the OCD function. That is, sincethe internal data outputted from the memory device via the levelshifters is inputted to a new circuit arrangement, that is, the OCDpre-driver, in accordance with the DDR II technique, so that theinternal data is processed in accordance with the function of the OCDpre-driver, an increase in current load occurs inevitably. In order tocope with the increased current load, the level shifters inevitably usePMOS and NMOS transistors having increased capacities and sizes.However, such increased capacities of the PMOS and NMOS transistors maycause an excessive increase or decrease in slew rate in the outputdriver. For this reason, there may be a problem in that it is impossibleto satisfy upper and lower limits required for desired characteristicsof products in association with slew rate. In particular, such a problemis more remarkable when an increase in slew rate occurs due to increasedcapacities of the PMOS transistors of the level shifters while exceedingthe upper limit required for desired characteristics in association withslew rate.

The following Table 1 describes measured values of slew rate in theconventional output driver circuit. Referring to Table 1, it can be seenthat the measured maximum values exceed 4.5 V/ns, which is an upperlimit prescribed in association with slew rate in specificationsprescribing requirements of characteristics. It can also be seen that,although the measured minimum values are not below 1.5 V/ns, which is alower limit prescribed in association with slew rate in thespecifications, it is approximate to the lower limit, so that thecircuit exhibits very unstable operation characteristics.

TABLE 1 Slew Rate Minimum Value Typical Value Maximum Value Up-Slew-Rate 1.5 Vn/ns 3.1 Vn/ns 5.1 Vn/ns Down-Slew-Rate 1.65 Vn/ns 3.1 Vn/ns 5.7Vn/ns

SUMMARY OF THE INVENTION

Therefore, it is an object to provide an output driver circuit for asemiconductor memory device, in particular, a memory device using a DDRII concept or a concept similar thereto, which can reduce a variation inthe slew rate of an output driver thereof between maximum and minimumvalues, while satisfying requirements of characteristics associated withslew rate.

In accordance with the present invention, this object is accomplished byproviding an output driver circuit for outputting internal data of amemory device to the outside of the memory device, comprising: apre-driver for generating a plurality of data signals, based on theinternal data; an off-chip driver (OCD) pre-driver for receiving firstand second ones of the data signals from the pre-driver andpredetermined control signals from a control unit included in the memorydevice, thereby generating a plurality of up-driving control signals forcontrol of up-driving impedance and a plurality of down-driving controlsignals for control of down-driving impedance; a push-pull driver forreceiving third and fourth ones of the data signals from the pre-driverand the up-driving and down-driving control signals from the OCDpre-driver, thereby generating an output signal; and a slew rate controlunit for controlling the internal data inputted the pre-driver to adjusta slew rate of the output signal.

The memory device may be a double data rate (DDR) II synchronous dynamicrandom access memory (SDRAM) (DDR II SDRAM).

The push-pull driver may comprise: a plurality of pull-up elements fordriving a voltage level of the output signal to a first output voltagelevel, based on the third data signal, the pull-up elements beingindividually and selectively driven, based on the up-driving controlsignals, respectively; and a plurality of pull-down elements for drivingthe voltage level of the output signal to a second output voltage level,based on the fourth data signal, the pull-down elements beingindividually and selectively driven, based on the down-driving controlsignals, respectively.

The memory device may be a DDR II SDRAM.

The slew rate control unit may be a resistor device.

The resistor device has a resistance of 300 to 400Ω.

The resistor device may be a plate type resistor device made of amaterial having a surface resistivity of 50 to 150 Ω/μm².

The resistor device may comprise an active resistor or a gate resistor.

The resistor device may comprise resistance components on bit lines ofthe memory device.

The output driver circuit having the above configuration operates asfollows.

A data output (Dout) signal, which is internal data of the memorydevice, is inputted to the slew rate control unit after being driven toa predetermined voltage level by level shifters. The Dout signal isdropped in voltage level by the slew rate control unit, and is thenapplied to the pre-driver.

Based on the Dout signal received from the slew rate control unit, thepre-driver outputs a plurality of data signals. Two of the data signalsare inputted to the gates of pull-up driving elements included in thepush-pull driver and the gates of pull-down driving elements included inthe push-pull driver, respectively. Another two of the data signals aresupplied to the OCD pre-driver. In addition to the latter two datasignals, the OCD pre-driver receives control signals generated from acontrol unit of the memory device in accordance with coding informationof the internal data. Based on the received signals, the OCD pre-drivergenerates a plurality of up-driving control signals for control ofup-driving impedance, and a plurality of down-driving control signalsfor control of down-driving impedance. The push-pull driver receives theup-driving and down-driving control signals from the OCD pre-driver,along with the data signal from the pre-driver. Based on the receivedsignals, the push-pull driver generates an output signal having apredetermined strength. The slew rate control unit is adapted to dull orcontrol a gradient of the Dout signal. The slew rate control unit may beimplemented, using passive or active resistor elements or a separatecontrol circuit adapted to dull a gradient of an output signal. The slewrate control unit prevents an abrupt variation in the gradient of theoutput signal, and thus, an increased variation in slew rate, caused byincreased capacities of PMOS and NMOS transistors included in the levelshifters adapted to supply the internal data after performing a voltagelevel transition for the internal data. Accordingly, the slew ratecontrol unit prevents the slew rate of the output signal from varyingbeyond a desired range meeting requirements of characteristics inproducts.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects, and other features and advantages of the presentinvention will become more apparent after reading the following detaileddescription when taken in conjunction with the drawings, in which:

FIG. 1 is a block diagram illustrating an output driver circuitaccording to an embodiment of the present invention;

FIG. 2 is a circuit diagram for explaining operations of a slew ratecontrol unit and a pre-driver, which are included in the output drivercircuit;

FIG. 3 is a circuit diagram illustrating a circuit configuration of theslew rate control unit;

FIG. 4 is a circuit diagram illustrating a circuit configuration of anOCD pre-driver pull-up unit included in the output driver circuit;

FIG. 5 is a circuit diagram illustrating a circuit configuration of anOCD pre-driver pull-down unit included in the output driver circuit; and

FIG. 6 is a circuit diagram illustrating a push-pull driver included inthe output driver circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail with reference tovarious embodiments thereof, which are made only for illustrativepurposes. The present invention is not to be construed as being limitedto those embodiments.

FIG. 1 illustrates an output driver circuit according to an embodimentof the present invention. Hereinafter, the output driver circuit will bedescribed in detail with reference to FIG. 1.

As shown in FIG. 1, the output driver circuit according to theembodiment of the present invention includes a slew rate control unit110 for receiving a data output (Dout) signal, which is internal data ofa memory device, to which the output driver circuit is applied, anddriven to a predetermined voltage level by level shifters (not shown),and outputting the received Dout signal, a pre-driver 120 for outputtingdata signals pullup_A, pulldown_A, pullup_B, and pulldown_B in responseto the Dout signal outputted from the slew rate control unit 110, and anOCD pre-driver 130 for receiving the signals pullup_A and pulldown_Afrom the pre-driver 120 while receiving control signals ocdpu_70,ocdpu_75, . . . , ocdpu_135, ocdpu_140, ocdpd_70, ocdpd_75, . . . ,ocdpd_135, and ocdpd_140 from a control unit (not shown) included in thememory device, thereby generating a plurality of down-driving controlsignals u70, u75, . . . , u135, and u140 for control of up-drivingimpedance, and a plurality of down-driving control signals d70, d75, . .. , d135, and d140 for control of down-driving impedance. The outputdriver circuit also includes a push-pull driver 140 for receiving thecontrol signals pullup_B, and pulldown_B from the pre-driver 120, andthe up-driving control signals and down-driving control signals from theOCD pre-driver 130, thereby generating an output signal.

The OCD pre-driver 130 includes an OCD pre-driver pull-up unit 131 foroutputting the up-driving control signals, and an OCD pre-driverpull-down unit 132 for outputting the down-driving control signals.

Now, operation of the output driver circuit having the above-describedconfiguration according to the embodiment of the present invention willbe described.

As shown in FIG. 2, a Dout signal, which is internal data driven from avoltage level VDD to a voltage level VDDQ by level shifters 201 and 202,is first inputted to the slew rate control unit 110. The slew ratecontrol unit 110 is adapted to dull or control a gradient of the Doutsignal. The slew rate control unit 110 may be implemented, using aresistor circuit or a separate control circuit adapted to dull agradient of an output signal. In particular, the slew rate control unit110 may be composed of resistors R1 and R2 having a resistance of 300 to400Ω. In this case, passive resistors, active resistors, gate resistors,etc. may be used for the resistors R1 and R2. The used resistors may beimplemented in the form of a plate type resistor, using a materialhaving a surface resistivity of 50 to 150 Ω/μm², in order to reduce thearea of the resistors occupied in the semiconductor memory device whilesatisfying the above-described required resistance. Alternatively, theresistors may be implemented, using resistance components on bit linesof the associated DRAM. The resistances of the resistors R1 and R2 maybe independently determined, depending on the environment of a system,to which the output drive circuit is applied.

In particular, where the semiconductor memory device, to which theoutput driver circuit is applied, is a DDR II SDRAM, the slew ratecontrol unit 110 prevents a considerable increase or decrease in voltageincrease rate, that is, slew rate, caused by increased capacities ofPMOS and NMOS transistors included in the level shifters 201 and 202.That is, the slew rate control unit 110, which is composed of resistors,is arranged between the level shifters 201 and 202 and the pre-driver120, to add impedance components. In accordance with such an addition ofimpedance components, it is possible to prevent the slew rate of theoutput signal from being susceptible to an increase in the capacity ofeach resistor, and thus, to prevent the slew rate of the output signalfrom varying beyond a desired range. Such an effect is more remarkablein preventing the problem encountered in DDR II products, that is, anincrease in slew rate exceeding an upper limit thereof.

Since the output driver circuit according to the embodiment of thepresent invention employs the slew rate control unit 110 as describedabove, it sufficiently satisfies requirements of characteristicsprescribed in specifications in association with upper and lower slewrate limits. Accordingly, it is possible to prevent an excessiveincrease in data skew and noise current.

Thereafter, the Dout signal outputted from the slew rate control unit110 is applied to the pre-driver 120, which in turn performs inversionor non-inversion processing for the Dout signal, thereby outputting datasignals pullup_A, pulldown_A, pullup_B, and pulldown_B.

As shown in FIG. 6, the data signals pullup_B and pulldown_B are appliedto PMOS and NMOS transistors P0 and N0 included in the push-pull driver140, as control signals to pull up and pull down an output signal,respectively.

The data signal pullup_A is inputted, as a control signal, to logicelements composing the OCD pre-driver pull-up unit 131 of the OCDpre-driver 130. As shown in FIG. 4, the OCD pre-driver pull-up unit 131not only receives the control signal pullup_A at first inputs of NANDgates thereof, but also receives, at respective second inputs of theNAND gates, control signals ocdpu_70, ocdpu_75, . . . , ocdpu_135, andocdpu_140 from a control unit (not shown) included in the memory device.The OCD pre-driver pull-up unit 131 performs a logic operation for thereceived control signals, thereby generating up-driving control signalsu70, u75, . . . , u135, and u140 for control of up-driving impedance,which is in turn sent to the push-pull driver 140.

The data signal pulldown_A is also inputted, as a control signal, tologic elements composing the OCD pre-driver pull-down unit 132 of theOCD pre-driver 130. As shown in FIG. 5, the OCD pre-driver pull-downunit 132 not only receives the control signal pulldown_A at first inputsof NOR gates thereof, but also receives, at respective second inputs ofthe NOR gates, control signals ocdpd_70, ocdpd_75, . . . , ocdpd_135,and ocdpd_140 from the control unit (not shown) included in the memorydevice. The OCD pre-driver pull-down unit 132 performs a logic operationfor the received control signals, thereby generating down-drivingcontrol signals d70, d75, . . . , d135, and d140 for control ofdown-driving impedance, which is in turn sent to the push-pull driver140.

The push-pull driver 140, which includes a plurality of pull-up-drivingPMOS transistors P0, P70, P75, . . . , P135, and P140, and a pluralityof pull-down-driving NMOS transistors N0, N70, N75, . . . , N135, andN140, receives the control signals pullup_B and pulldown_B from thepre-driver 120, while receiving the up-driving control signals anddown-driving control signals from the OCD pre-driver 130. Based on thereceived signals, the push-pull driver 140, which is an output driver,generates an output signal.

Now, the principle of adjusting the impedance of the output driver inaccordance with the embodiment of the present invention will bedescribed.

When the internal data of the memory device, that is, the Dout signal,has a high level, the signals pullup_B and pulldown_B, which areoutputted from respective inverters INV1 and INV2 of the pre-driver 120,have a low level, so that they turn on the PMOS transistor P0 of thepush-pull driver 140 while turning off the NMOS transistor N0 of thepush-pull driver 140, respectively. As a result, a high-level signal isoutputted from an output terminal of the push-pull driver 140. In thiscase, the control signals ocdpu_70 to ocdpu_100 and ocdpd_70 toocdpd_100 outputted from the control unit (not shown) included in thememory device are basically set to have a high level, whereas thecontrol signals ocdpu_105 to ocdpu_140 and ocdpd_105 to ocdpd_140outputted from the control unit are basically set to have a low level.

Accordingly, where the internal data, that is, the Dout signal, has ahigh level, the control signals u70 to u100 outputted from the NANDgates of FIG. 4, to which the signals ocdpu_70 to ocdpu_100 are inputtedin a high-level state, have a low level because the signal pullup_A hasa high level, so that they turn on the PMOS transistors P70 to P100 ofthe push-pull driver 140 shown in FIG. 6, respectively. On the otherhand, the control signals u105 to u140 outputted from the NAND gates ofFIG. 4, to which the signals ocdpu_105 to ocdpu_140 are inputted in alow-level state, have a high level, so that they turn off the PMOStransistors P105 to P140 of the push-pull driver 140 shown in FIG. 6,respectively. In this case, the NOR gates of the OCD pre-driverpull-down unit 132 receive the high-level signal pulldown_A atrespective first inputs thereof, so that the control signals d70 to d140outputted from respective NOR gates have a low level. As a result, theNMOS transistors N70 to N140 of the push-pull driver 140 are turned off.Thus, only the PMOS transistors P70 to P100, which are pull-up drivingelements of the push-pull driver 140, are turned on to pull up an outputsignal at the output terminal of the output driver circuit.

In this case, where it is necessary to adjust the impedance of theoutput driver, this adjustment can be achieved by selectively adjustingrespective voltage levels of the control signals ocdpd_70, ocdpd_75, . .. , ocdpd_135, and ocdpd_140 to a high or low level, thereby selectivelyadjusting respective pull-up-driving control signals of the associatedPMOS transistors of the push-pull driver 140 to a high or low level,that is, adjusting the number of the selectively-turned-on PMOStransistors.

Where the internal data, that is, the Dout signal, has a low level, theimpedance adjustment of the output driver is achieved by adjusting thenumber of the selectively-turned-on NMOS transistors of the push-pulldriver 140 in accordance with an operation similar to that of the abovedescribed case.

As described above, the slew rate control unit 110 in this embodimentprevents a considerable increase or decrease in slew rate caused byincreased capacities of the PMOS and NMOS transistors included in thelevel shifters 201 and 202, in particular, in the case in which theoutput driver circuit of this embodiment is applied to a DDR II SDRAM.That is, the slew rate control unit 110 is arranged between the levelshifters 201 and 202 and the pre-driver 120, to add impedancecomponents. In accordance with the addition of impedance components, itis possible to prevent the slew rate of the output signal from beingsusceptible to an increase in the capacity of each resistor whilereducing such a variation in slew rate between maximum and minimumvalues, thereby preventing the slew rate of the output signal fromvarying beyond a desired range.

As apparent from the above description, in the output driver circuitaccording to the present invention, the slew rate control unit isarranged at an upstream node of the pre-driver to control the slew rateof the output signal to meet requirements associated with upper andlower slew rate limits. Accordingly, it is possible to prevent anexcessive increase in data skew and noise current. Thus, the presentinvention makes it possible to produce semiconductor memory deviceshaving a sufficient competitiveness.

Although the preferred embodiments of the invention have been disclosedfor illustrative purposes, those skilled in the art will appreciate thatvarious modifications, additions and substitutions are possible, withoutdeparting from the scope and spirit of the invention as disclosed in theaccompanying claims.

1. An output driver circuit for outputting internal data of a memorydevice to the outside of the memory device, comprising: a pre-driver forgenerating a plurality of data signals, based on the internal data; anoff-chip driver (OCD) pre-driver for receiving first and second ones ofthe data signals from the pre-driver and predetermined control signalsfrom a control unit included in the memory device, thereby generating aplurality of up-driving control signals for control of up-drivingimpedance and a plurality of down-driving control signals for control ofdown-driving impedance; a push-pull driver for receiving third andfourth ones of the data signals from the pre-driver and the up-drivingand down-driving control signals from the OCD pre-driver, therebygenerating an output signal; and a slew rate control unit forcontrolling the internal data inputted the pre-driver to adjust a slewrate of the output signal.
 2. The output driver circuit according toclaim 1, wherein the memory device is a double data rate (DDR) IIsynchronous dynamic random access memory (SDRAM) (DDR II SDRAM).
 3. Theoutput driver circuit according to claim 1, wherein the push-pull drivercomprises: a plurality of pull-up elements for driving a voltage levelof the output signal to a first output voltage level, based on the thirddata signal, the pull-up elements being individually and selectivelydriven, based on the up-driving control signals, respectively; and aplurality of pull-down elements for driving the voltage level of theoutput signal to a second output voltage level, based on the fourth datasignal, the pull-down elements being individually and selectivelydriven, based on the down-driving control signals, respectively.
 4. Theoutput driver circuit according to claim 3, wherein the memory device isa DDR II SDRAM.
 5. The output driver circuit according to claim 1,wherein the slew rate control unit is a resistor device.
 6. The outputdriver circuit according to claim 5, wherein the resistor device has aresistance of 300 to 400Ω.
 7. The output driver circuit according toclaim 5, wherein the resistor device is a plate type resistor devicemade of a material having a surface resistivity of 50 to 150 Ω/μm². 8.The output driver circuit according to claim 5, wherein the resistordevice comprises an active resistor or a gate resistor.
 9. The outputdriver circuit according to claim 5, wherein the resistor devicecomprises resistance components on bit lines of the memory device.